An inductive power supply may be used to supply wireless power to power or charge secondary devices. In some known inductive power supplies, secondary devices are powered or charged by placing them on a charging surface. Many inductive power supplies limit spatial freedom by requiring specific placement and orientation of the remote device with respect to the inductive power supply.
In some known inductive power supply systems, a single primary coil 102 is embedded in a charging surface 104 of a charging device 106 and a single secondary coil 108 is embedded in a secondary device 110. For example, in the prior art inductive power supply system shown in FIGS. 1 and 2 one secondary coil 108 is embedded in the secondary device 110 and is aligned in close proximity to the primary coil 102 embedded in the charging device 106. Power is provided from a mains input to the charging device 106, sometimes referred to as a wireless power supply. Assuming the mains input provides AC power, the power can be rectified in a mains rectification circuit 202 into DC power and then can be adjusted in a DC/DC power supply 204. An inverter 206 can switches the DC power at a frequency controlled by the controller 208 in order to generate an AC signal across the inductive tank circuit 210 to produce an electromagnetic field. The tank circuit in most conventional inductive power supplies includes a primary coil 102 and a primary capacitor 213 The secondary device 110 includes a secondary coil 108 and an optional resonant capacitor 214 to receive the electromagnetic energy. The AC signal can be rectified into DC power in a rectification circuit 216. From there, the DC power can directly power the load 220, or where the load is a battery the power can be used to charge the battery. A controller 218 may be utilized to control how the power is applied to the load or to control a charging algorithm for charging a battery. In this type of system, power transfer efficiency is typically increased when the coils are aligned center to center, and when the spacing between the primary and secondary coils is reduced. However, this requirement of close one-to-one alignment in order to effectively communicate and transmit power restricts spatial freedom and limits the charger to operation with one secondary device at a time. To enable a surface with wireless power, the user is typically provided with information about where the device needs to be located. This is usually done with a magnetic alignment feature, or with different mechanical guides that force devices to be placed in a certain place, or with graphical elements that guide the user to correctly place the device. Some users would like more freedom to move the secondary device around on the surface of the charging device.
Some solutions to this problem have been proposed. For example, U.S. patent application Ser. No. 12/652,077 to Baarman et al, filed on Jan. 5, 2010 discloses an inductive power supply with a movable coil and is herein incorporated by reference in its entirety. The moveable coil is one mechanical solution to achieve the desired spatial freedom over the surface of the charger while maintaining close coil proximity. The moving coil solution can increase spatial freedom but can introduce the risk of potential mechanical reliability problems.
Another proposed solution is to utilize a large primary coil so that energy can be provided over a greater area. This solution can be problematic because a larger coil may create undesired stray magnetic fields and it can be difficult to transfer power efficiently.
Yet another proposed solution is to provide an array of coils arranged adjacently in a single layer. In this solution, a number of primary coils are disposed in an array near the charging surface. When a device is placed on the charging surface that is greater in size than the device, energy is only transferred from that part of the planar charging surface that is directly beneath the device, and possibly immediately adjacent areas that are able to couple to the secondary coil. That is, in one configuration, all of the coils of the array of primary coils are driven simultaneously to create magnetic flux that is substantially uniform over the charging surface so that the precise position and orientation of the electronic device on the charging surface is not critical. In addition, parasitic loads, such as pieces of metal or non-wirelessly powered devices, can absorb the magnetic field and lower the system efficiency.
Some solutions propose a multi-layer coil array in order to provide a more uniform magnetic field distribution. One problem with a single layer array of coils is that where there are gaps between the coils, the magnetic field is lower, which is sometimes referred to as a valley. By having two or more layers of coils arranged such that the center of a winding pattern on one layer is placed on the gap between adjacent winding patterns on the other layer, a more uniform field distribution can be achieved. Energizing all those coils simultaneously can lead to hot zones and dead zones due to field construction and field deconstruction effects that occur from overlapping fields. In addition, parasitic loads, such as pieces of metal or non-wirelessly powered devices, can absorb the magnetic field and lower the system efficiency.
Some array solutions attempt to circumvent having to turn on a large amount of coils by providing magnetic attractors to specifically locate the device on a charging surface so that power can be transferred utilizing a single coil. However, magnetic attractors add cost, complexity, and can lower efficiency of the power transfer system. Various ergonomic alignment solutions have also been proposed, but these aids can disrupt the aesthetics of surfaces, add complexity to the design of the surface, and can affect the usability because alignment still may not be guaranteed.